1. Field of the Invention
This invention relates to excavators and, more particularly, to hand-held pneumatic excavators.
2. Description of the Prior Art
Since the 1960's, contractors have used compressed air exiting from the end of an open pipe for cleaning operations. Such uses include loosening soil from around buried water pipes, gas mains, electrical cables and the like in small hole excavations. Previously, picks, digging bars, spades, buckets or blades having hard cutting edges were used to uncover a buried object and often caused significant damage to the buried object. The use of compressed air has the advantage of pulverizing most soil types without damaging the buried object.
Pneumatic devices which use compressed air to uncover buried objects typically include a source of air under pressure, such as a compressor connected via a valve mechanism to an elongated tube. The valve mechanism controls the flow of compressed air through the tube.
However, compressed air exiting from the open end of a tube expands suddenly to atmosphere in an unfocused, complicated and wasteful manner. To improve performance of compressed air excavators, a nozzle is incorporated on the exit end of the tube. Typically, the nozzle has an inwardly converging upstream end which merges into a diverging downstream end. This configuration acts to reduce the pressure of the air or gas and increase its velocity. European Patent No. 0 251 660 describes a typical pneumatic hand tool based upon this design. U.S. Pat. No. 5,170,943 shows a similar high velocity pneumatic excavating hand tool having a valve connecting a pressurized gas supply to a non-conductive tube having a terminal nozzle.
However, little attention has been paid to the proper engineering design of the nozzle for excavation purposes. Excavation results appear to be empirically based; some prior art devices supposedly provide results which do not correspond to gas dynamic relations governing the performance of these nozzles. To achieve the air exit velocities cited in one of these latter devices, it would have to be supplied with compressed air at over two hundred psig (pounds per square inch gauge). This is well over double the one hundred psig normally used in the trade and generally available from a conventional portable air compressor. Previous devices, hence, have been energy inefficient and wasteful.
In addition, these prior art devices generate a large amount of dust and debris blowback at the operator during digging operations. This is due both to an over use of compressed air and the unfocused and highly dissipative nature of the nozzles. In an extreme case, standing shock waves may be formed downstream of the nozzle exit. These shock waves suddenly decelerate the air jet speed from supersonic to subsonic with concomitant loss of excavation ability.
Conventional portable air compressors which serve as the gas supply for prior art devices are constrained because the range of the effective working distances of the prior art devices is limited by the length of an air hose from the air compressor, generally no more than fifty to one hundred feet. Because conventional portable air compressors generally must be towed into position by other vehicles, they are limited to roadways and to areas adjacent to level or smooth areas capable of being traversed by vehicles. Farm lands, fields, water holes, forested or rocky areas are difficult to reach effectively in this manner.
Since the late 1970's, there has been considerable work towards developing small, inexpensive, portable, high flow air units. These are typically used to blow away leaves, grass clippings and the like from in front of an operator. U.S. Pat. No. 4,288,886 discloses an air broom system which includes a housing, a portable power unit, a rotary impeller unit and a tube from which air exits. U.S. Pat. No. 5,195,208 discloses a backpack power blower apparatus made up of a frame supporting an engine, a blower and associated parts, including sound insulation. Although these prior art devices can be carried on an operator's back and produce an airstream, none produces an airstream at a rate which is sufficient to excavate material more cohesive than loose sand. None of these prior art devices contains a compressor to generate such an airstream.
The ability to safely unearth a variety of buried objects is becoming increasingly important, especially in public and private remediation of hazardous waste sites. From the industrial and nuclear energy sector, such objects include glass bottles, cardboard or wooden boxes, metal or fiber drums and metal cylinders containing chemical or radioactive hazardous waste. Backhoes or hydraulic excavators are the most commonly used machines for such excavating. However, excavation using heavy machinery is slow because of the extra time and care required to maneuver around hazardous buried objects. Despite the care with which they might be used by a skilled operator, a risk exists that the hard cutting edges of heavy machinery may cause significant damage to the buried object. Government and industry are beginning to mandate only hand digging in an area with suspected buried objects. Hand digging is slow, fatiguing and costly in both direct manpower and idling of heavy excavating machinery.
Buried objects from the military sector, such as unexploded ordnance, chemical munitions and mines, require particular attention. Presently, an operator dressed in a flak jacket with helmet and face shield searches an area with a mine detector until a buried object is located. The operator then must very carefully dig up the object, usually with a trowel or knife, until it is fully uncovered and identifiable. The mine is then disabled. This procedure must be followed for every detector indication, whether erroneous or true. Detection and removal of mines is extremely dangerous, especially when carelessness due to operator fatigue sets in. In typical land mine cleanup operations, it has been reported that for every mine uncovered and deactivated, up to one thousand false hits can be recorded by the mine detector being used. All of these must be carefully excavated by hand. Assuming one minute per excavation, more than sixteen hours can be spent digging up false hits for each live mine found.
In addition, plastic mines and unexploded ordnance usually remain hidden unless they have enough internal metal to trigger a detector. Detection of plastic can be done using subsurface radar. Such systems, however, are expensive, bulky to deploy and difficult to interpret.
A mine plow disclosed in U.S. Pat. No. 5,291,819 provides for delivery of a high velocity gas stream to the ground surface near the mine plow's baskets. The gas stream winnows loose soil material from metal and plastic battlefield debris which has been picked up by the baskets. The mine plow is limited to previously uncovered objects and does not achieve primary excavation of the battlefield debris.
A need remains for a highly mobile, person transportable pneumatic excavator, which can safely uncover buried objects.
Therefore, it is an object of the present invention to provide a pneumatic excavator which can uncover buried objects and, in particular, safely uncover unexploded ordnance.